When Depa was very young, her family left their home planet of Chalacta in search of a new place to live. While traveling, the transport they were on was attacked by pirates, and in the ensuing fight Depa's parents were killed. The infant girl and her sister, however, were saved from the space pirates by the respected Jedi Master Mace Windu. He brought the six-month-old force-sensitive child to the Jedi Temple, and, when she was of age, took her as his Padawan. Windu was the creator of the lightsaber combat form Vaapad. Windu taught her all he knew, and she became as close to him as a daughter. Depa's sister was also brought to the Temple, and given the name Sar Labooda.
Depa embraced the teachings of the Chalactan Adepts, making her attuned to the minds and personalities of her peers. Decorating the bridge of her nose were the Chalactan Greater and Lesser Marks of Illumiation, a testament to her spirituality.
Known for her wisdom, bravery, and loyalty, Depa was widely considered to be one of the most enlightened Jedi of her time. As such, she became one of the youngest Jedi ever to be appointed to the High Council, selected around 33 BBY. Shortly before the Invasion of Naboo, Depa was dispatched to Asmeru to investigate an attempt on the life of Supreme Chancellor Valorum. While on the planet, she and her fellow Jedi were attacked by Nebula Front terrorists, but fended them off.
Like her sister Sar Labooda, Billaba was a soft-spoken spiritual individual, but capable enough to take quick action if need be. A consummate lightsaber duelist, she mastered the Juyo form of lightsaber combat, and was also one of only two Jedi to master Vaapad, a series of techniques that was widely regarded to complete the Juyo lightsaber form, but also posed a grave spiritual threat to its wielder.
In 32 BBY, Depa, along with the other Jedi Masters on the Council, was confronted by Jedi Master Qui-Gon Jinn and his apprentice Obi-Wan Kenobi. She watched as Qui-Gon requested that young Anakin Skywalker be tested for admission into the Jedi Order. According to Jinn, the boy was the Chosen One of the Jedi prophecy. After testing the boy, the Council decided that the boy was not to be trained due to the emotional attachment to his mother. Following the Invasion of Naboo, Billaba attended the funeral of the maverick Jedi and participated in the festivities that followed.
Shortly after the discovery of young Skywalker and the death of Qui-Gon, Depa embarked on a mission to rescue her former Master, Mace Windu, who had gone alone to Nar Shaddaa to investigate an animal smuggling ring. Journeying to the Smuggler's Moon, Depa found Mace and reached him just as he was being surrounded by a large number of underworld thugs. After aiding him against the thugs, she helped him with his mission. They had saved the alewoman Aunuanna who claimed to have seen another Jedi that day, just then Quinlan Vos appeared and told them to complete their mission they would have to go to the bottom-most regions of Nar Shaddaa. Aunuanna helped the Jedi by giving them her daughter's father's speeder saying it was the only way to reach the depths safely. On the way down the two Jedi were attacked by Vrblthers, Depa lost control of the speeder and they crashed into the Circus Horrificus. Here they confronted Gargonn the Hutt, who unleashed deranged Akk dogs into the arena. Mace and Depa killed an akk by stabbing it through the eyes at the same time. They then jumped up to Gargonns balcony and forced him to show them the manifesto's of the Circus Horificus. Depa believed they could force enough influence on Nar Shaddaa to stop the violation of the Non-Sentient Creatures Transportation Act.
When Mace Windu and 212 other Jedi journeyed to Geonosis to rescue Obi-Wan Kenobi, his apprentice Anakin Skywalker, and Senator Padme Amidala from execution by the Separatists, Depa stayed at the Jedi Temple to safeguard the younglings while the other Jedi were away. Although she possessed considerable fighting prowess, Depa was chosen to stay at the Temple as she was one of the principal instructors of young Jedi clans, such as the Bear Clan.
Not long after the Battle of Geonosis, which marked the start of the Clone Wars, Depa was dispatched to Mace Windu's home planet of Haruun Kal to stop Separatist activity and possibly recruit Korunnai militia to aid the Republic. She became a leader of the Upland Liberation Front (ULF), along with Kar Vastor, but was tormented by nightmares and by the atrocities she had witnessed in the brutal jungle war. Though her nature seemed to be perfectly suited for this test, Billaba turned to the dark side of the force in 22 BBY due to the horrific nature of the guerrilla war and the influence of Kar Vastor. Eventually, a message composed by Depa was delivered to Yoda and Mace Windu, who were quite concerned for Depa's condition.
Mace Windu journeyed to Haruun Kal and tried to save her, but Depa was weakened under the influence of Vastor, and had committed war crimes while leading the ULF. Significantly, her Chalactan Mark of Greater Illumination had been torn out somehow, signifying a lack of spiritual strength and clarity to Mace. Mace took himself hostage in order to reach her, after enduring a series of ordeals and attacks to reach her. He arrested her and had intended to take her to safety, but a massive militia attack forced Depa into combat, which was the peak of her final turn to the dark side.
Defending against the militia, Mace, Depa, and some of the Korunnai decided to take the fight to the enemy and hijacked enemy gunships. Receiving aid from clone trooper reinforcements, Depa and Mace led an assault on the main militia base. After seizing the control bunker in the capital city of Pelek Baw, Depa, Vastor, and his Akk Guards sealed themselves inside and waited for Mace Windu to return from his mission to find Lord Geptun, the leader of the militia.
Having found and brought Geptun back with him, Mace found Depa and believed she was dead, but she was faking and attacked him, as the clone troopers battled Vastor and his Akk Guards. Mace later said she would have won the following duel easily, but quick thinking on his part caused her to inadvertently destroy the droid starfighters that were creating fear, despair and anger—which she had been feeding off of. Realizing what she had done, Depa attempted to take her own life, and, although prevented from doing so by Nick Rostu, she remained in a coma for the rest of the war.
The Council seat she left vacant after she fell into a coma was eventually given to Obi-Wan Kenobi along with the title of Jedi Master. Depa's fate later is unknown, but as she was still in the coma during the Great Jedi Purge, her chances of having survived the raid on the Jedi Temple were very slim.